Recently, electronic devices are increasingly required to have high reliability and to be small and light. Electronic elements such as transistors, diodes, IC, resistors, and capacitors, electronic components such as connectors, and printed circuit boards are bonded with tin or solder to form an electronic circuit, which is widely used as a semiconductor device or an electronic device. In accordance with the reduction in size of these electronic elements, electronic components, and printed circuit boards, the solder bonding portion is getting minute and thus is required to have higher quality and reliability than ever. Particularly, extremely strict reliability is required in the quality of the minute solder bonding of printed circuit board and electronic elements.
For this reason, bonding strength is required also in the tin or solder alloy used for tin or solder bonding. Particularly, high reliability is required of a minute tin or solder bonding portion according to the miniaturization of pitch spacing and bonding area of the leads of electronic elements or electronic components.
On the other hand, recently, the prohibition or restriction of use of lead has been promoted due to problems of environmental pollution and harmful effects to the human body. Particularly, in the field of electronic components, so-called “lead-free solder alloy” containing no lead has been widely used in the soldering process. In particular, tin-silver-copper based solder alloy and solder alloy additionally added with antimony (Patent Document 1), solder alloy formed by adding nickel, germanium, and the like to tin-silver-copper based solder alloy (Patent Document 2) and the like have been proposed and put to practical use. In addition, various kinds of solder alloy have been proposed such as tin-zinc-nickel based solder alloy, and solder alloy additionally added with silver, copper, bismuth, and the like (Patent Document 3).
Generally, the method of bonding tin or solder alloy to a copper land surface or a copper and copper alloy lead surface of electronic components such as printed circuit boards, semiconductor packages, and electronic elements is as follows:
Tin or solder alloy is heated at a temperature equal to or higher than the melting point in a reservoir so that it is melted to get liquid. Then, the tin or solder liquid is supplied by a pump and jetted from a nozzle or a slit to the copper land or copper lead on the surface of a material such as a printed circuit board, electronic components or the like that is to be tin or solder bonded. The overflowing liquid is brought back to the original reservoir and is used in circulation.
However, at the time of soldering, flux or copper oxide film or the like on the surface of the copper land or the copper lead portion is mixed into the overflowing molten tin liquid or molten solder alloy liquid as impurities. This results in that the copper concentration or impurity concentration of the molten tin or the molten solder alloy liquid gets higher and higher with the passing of time. In addition, the solder composition changes, and not only the physical properties but also the chemical properties of the solder change.
Then, the solderability deteriorates particularly, and thus the solder forms with its greater volume than necessary so-called “excess volume” (horn, icicle) at the bonding portion. In a circuit having a small pitch spacing between leads, there are drawbacks such as that leaks easily occur through bridges formed toward adjacent leads. Besides, an outward phenomenon such as incomplete soldering occurs (Patent Document 4).
In addition, the elongation, which is one of the physical and mechanical characteristics of bonded solder, becomes smaller. Accordingly, after an electronic circuit is mounted on a semiconductor device or the like and repeatedly turned on and off, the solder bonding portion is fatigue-broken due to the heat cycle to easily cause conduction failure. It is widely known that this impairs the connection reliability of minute electronic devices.
Accordingly, after continuous soldering process, when the copper concentration exceeds a certain upper limit and the above-described solder characteristic deteriorates to cause an abnormal phenomenon (quality inferiority), it is common that the deteriorated solder is relatively frequently discarded and the renewed solder is used. That is, efficiency is extremely poor from the viewpoint of resources, workability, quality stability, and economy.
For this reason, particularly, as a method for improving efficiency by suppressing the increase of copper concentration at the time of the continuous process of the printed circuit board and the like, a method has been proposed and put to practical use in which a second solder which mainly contains an alloy excluding copper is supplied when molten solder liquid in a reservoir is lowered to a certain liquid level during the continuous process.
However, while this method can suppress the increase of copper to some extent, the increasing tendency with the passing of time does not change. Accordingly, the quality of solder bonding deteriorates with the passing of time. In addition, it is cumbersome to prepare and manage the second solder. Thus, despite a merit of extending the duration of use, the method is not quite sufficiently satisfactory (Patent Document 4).
Patent Document 1: Japanese Patent Application Laid-Open No. 5-50286 (Japanese Patent No. 3027441)
Patent Document 2: Japanese Patent Application Laid-Open No. 11-77366 (Japanese Patent No. 3296289)
Patent Document 3: Japanese Patent Application Laid-Open No. 9-94688 (Japanese Patent No. 3299091)
Patent Document 4: Japanese Patent Application Laid-Open No. 2001-237536 (Japanese Patent No. 32216709)